1. Field of the Invention
The invention relates to a light emitting diode (LED) package structure, and more particularly to a high power LED package structure.
2. Description of the Related Art
Various LED package structures are designed to address various optical, electrical and thermal performance problems LEDs with higher input power. The three LED package structures in use, lamp type LED, piranha LED and high power LED, are classified according to input power. FIG. 1a shows a conventional lamp type LED package structure produced by Luxeon Corp. (Luxeon Reliability Application Brief AB25). The main packaging element of the conventional lamp type LED package structure comprises an LED chip 11, a die attachment (not shown), a wire bonding 12, a reflector cup 13, a lead frame 14 and a lens 15. The lens may have one of a variety of optical designs according to various optical requirements. The largest LED chip in a conventional lamp type LED is about 0.35×0.35 mm2, and the maximum input power is about 0.1 watts. Because the input power is low, the lamp type LED package structure can not address electrical performance and thermal problems. To meet LED brightness requirements, an LED package structure with higher input power is desirable. FIG. 1b shows a stereogram of a conventional piranha LED package structure. Compared with the lamp type LED, the input power of the piranha LED is about 0.2 to 0.5 watts. The main packaging element of the conventional piranha LED package structure comprises an LED chip (not shown), a die attachment (not shown), a wire bonding (not shown), a reflector cup (not shown), a lead frame 24 and a lens 25. The LED chip of the conventional piranha LED is about 0.35×0.35 mm2 to 0.61×0.61 mm2. The main packaging elements of the piranha LED package structure are substantially the same as the lamp type LED. The lead frame of the piranha LED package structure has four leads, two more than the lamp type LED. The lead frame of the piranha LED package structure provides a better heat dissipating path than the lamp type LED, improving thermal performance. The piranha LED package structure may be soldered on a printed circuit board (PCB) by pin through hole or surface mount technology. FIG. 1c shows a conventional high power LED package structure produced by Luxeon Corp. (Luxeon Reliability Application Brief AB25), with input power is about 1 to 5 watts. The main packaging element of the conventional high power LED package structure comprises an LED chip 31, a wire bonding 32, a lead frame 34, a plastic lens 25, a silicone encapsulant 36, a heat slug 37, and a plastic case 38. The LED chip of the conventional high power LED is greater at about 1.0×1.0 mm2. Electrical performance and thermal performance of the high power LED package structure must be considered. Unless using the packaging elements of the lamp type LED, a heat slug, must be included in the high power LED package structure. Other designs are also required to prevent electrical overstress (EOS) by electrostatic discharge (ESD).
Because of their differing package structures, processes for fabricating lamp type LED, piranha LED and high power LED package structures are different. In the lamp type piranha LED, injection molding first forms a lead frame, the reflector cup is then formed on the lead frame, and bending the lead frame forms the lead. The lead frame of the lamp type LED or the piranha LED is thus completely formed. Next, an LED chip is placed on the reflector cup. A wire bonding is subsequently bonded to the LED chip and the lead. The lead frame is then placed in a mold cup, the cup is then filled with epoxy to form a lens. After hardening by heat, the lens is removed from the mold cup, and excess parts are excised to form the lamp type LED or the piranha LED package structures. The aforementioned manufacturing method of the lamp type LED or the piranha LED package structures has advantages of high throughput and low cost. The high power LED package structure may comprise a heat slug for dissipating a heat generated from the LED chip. FIG. 1d shows a conventional high power LED package structure disclosed in U.S. Pat. No. 6,274,924 B1. The main packaging element of the conventional high power LED package structure, as shown in FIG. 1d, comprises a heat slug 410, a lead frame combined with a housing 412, a reflector cup 414, an LED chip 416, a submount 418, and a lens 420. For the conventional high power LED package structure as shown in FIG. 1d, a housing is combined with a lead frame to form lead frame combined housing 412 for insulation. The lead frame combined housing 412, the lens 420, the LED chip 416 and the heat slug 410 are constructed together to form the conventional high power LED package structure as shown in FIG. 1d. The heat slug of the high power LED package structure may be disposed by various methods. FIG. 1e shows a conventional high power LED package structure disclosed in U.S. Pat. No. 6,376,902 B1 and U.S. Pat. No. 2004/0238930 A1, and FIG. 1f shows a cross section taken substantially along line I-I′ of FIG. 1e. A heat slug 502 is directly combined with a lead frame 507 and a housing 503. A recess 511, in an inner side of the housing 503, may be coated with a reflective material to form a reflective cup. The conventional high power LED package structure as shown in FIG. 1e is a semi-finished product formed by directly combining the heat slug 502, the lead frame 507 and the housing 503.
High power LEDs in a lamp type LED package structure are an appropriate choice when no particular brightness requirements exist, due to the relatively lower manufacturing costs thereof. To balance LED brightness with manufacturing cost, high the piranha LED package structure is also an economical choice. The piranha LED package structure, however, suffers from poor thermal performance due to heat dissipation problems while using the high power LED. According to the document “Advanced thermal design of electronic equipment” by Rensburg Ralph, 1997, the reliability of electronic products is reduced by about 50% when the temperature of the LED chip increases by about 10° C. Additionally, the brightness and life of the LED chip are reduced when the LED chip temperature is increased.
A high power LED package structure with lower thermal resistance capable of improving reliability and preventing brightness decay of an LED chip is desirable.